bug in TI in430.h implementation

Back when I added intrinsics to mspgcc I got confirmation that CCS and
IAR both implicitly added the required NOP after a DINT instruction
generated with _disable_interrupts(). (The MSP430 instruction set
explains that this is necessary to eliminate the possibility of
interrupting a subsequent pipelined instruction.)

The implementation of _disable_interrupts() in in430.h from TI's
GCC_RH_20140508.zip does not include this required instruction, making
it inconsistent with all other MSP430 toolchains. (Fortunately,
msp430-elf does generate a warning in this situation.)

Interestingly, the implementation of _enable_interrupts()
unconditionally adds a NOP after EINT, even though that workaround is
arguably required only on certain devices with a specific erratum, and
this behavior is AFAIK also inconsistent with other toolchains.

msp430-elf should behave as close to CCS and IAR as possible so
developers don't have to work around toolchain-specific
inconsistencies in how these operations are implemented.

TI, please fix this for the next header release.

Please also add a monotonically increasing version number to in430.h
or iomacros.h so applications can work around bugs like this.